Braided structures are produced by interlacing strands (containing fibers) of braiding material to form a braided pattern. Each strand has a width measured by the distance between its outmost edges exposed on the surface of the braided structure. Each strand also has a thickness measured by the distance between the fibers on the outer and inner surfaces of the braided structure (or the distance along the axis perpendicular to the width). The width to thickness ratio of such strand is referred to as the aspect ratio. In addition, the braided structure has a coverage area. Depending on the width of the strands and the angle of the braided structure (also called the braiding angle), interlacing the strands can produce gaps where the strands cross each other. The coverage area can be expressed as the amount of the braided structure surface area which contains braiding material or fiber.
There are a myriad of uses for high coverage area braided structures produced by high width to thickness ratio braiding material. One example is a new generation of automotive inflatable safety devices, or airbags. Airbags are installed in automobiles in order to protect the vehicle's occupants from injury. During a collision, the occupants are often thrown, causing the head or upper body to hit objects in the passenger compartment. Rapidly inflating airbags prevent bodily contact with such objects.
Certain airbags have been developed to exploit a particular feature of braided structures, namely that such structures shrink in length as they expand in diameter. This feature, essentially the well-known Chinese finger trap effect in reverse, allows for airbags constructed from braided structures to be elongated and stowed in a vehicle and, upon inflation from an impact initiated gas generator, to shorten in length, making a straight line between their attachment points. This feature allows such airbags to self deploy into a position suitable to provide the occupants with an appropriately positioned protective cushion. One example of an appropriate position is for side impact head protection. For this use, elongated, deflated airbags are stowed above the vehicles side window between the roof and the interior trim and are connected generally to a first point near the base of the windshield and a second point above the rear passenger door. Upon inflation, such airbags simultaneously shorten in length and expand in diameter to provide a diagonally positioned side cushion across the adjacent door and window, between the door and window and the vehicle's front seat occupant.
For the airbag applications just described, a braided structure is placed over a gas tight inflatable bladder (this application for the braided structure is also referred to as an airbag cover). It is important that the structure have a high coverage area of fibers over the gas tight inflatable bladder. The high degree of coverage is essential to reinforce the bladder material in order to prevent it from popping or rupturing when initially inflated and when receiving additional loads resulting from impact. It is also important, in at least a side impact airbag application, for the braiding material to have a small thickness or thin cross section so that the deflated airbag can be stowed in the relatively small area between the roof and the interior trim.
As is known in the braiding art, braiding machines include numerous carriers, each of which hold a strand of braiding material. Each strand (also referred to as a tow bundle) can consist of, for example, a single multifilament textile fiber or a multifilament yarn (also referred to as a yarn end) which contains several multifilament textile fibers. In order to create relatively thin braided structures usable for applications such as airbags, yarn ends with a relatively low linear density are used. A relatively low linear density textile fiber has a denier value of less than or about 2000. However, multifilament textile fibers and, accordingly, a multifilament yarn ends made of up such fibers generally have a round cross-section. Therefore, the low linear density yarns employed to achieve relative thinness in the braided structure will make a correspondingly small contribution to the structure's coverage area. As such, in order to increase the coverage area, more yarn ends must be introduced into the braiding process.
This can be accomplished, for example, in two ways. First, many strands which each contain a single yarn end can be incorporated by using a relatively large braiding machine with numerous carriers. Second, multiple yarn ends can be combined in a single strand or tow bundle. This can be achieved by orienting the yarn ends for each strand side-by-side or parallel in a single layer relative one another. Accordingly, the width of the strand made up of a tow bundle is distance between the outer edges of the collection of yarn ends and its thickness is the thickness of any yarn end since the yarn ends are oriented in parallel. These two approaches can also be combined such that a braiding machine with a larger number of carriers can include an increased number of yarn ends on the strand of each carrier.
We have determined that such a high coverage area braided structure can be constructed using a thin braiding material on a 336-carrier braiding machine. The 336-carrier braiding machine is a relatively large machine, using three parallel yarn ends having 500-denier high tenacity polyester per carrier for a relatively thin, wide strand or tow bundle. However, there is a disadvantage to this construction in that large 336-carrier braiding machines are relatively expensive, rare, and relatively unproductive as a function of the fineness of the weave necessary to create a high coverage area braided structure. Likewise, the 500-denier polyester requires twisting in each of the three parallel yarn ends to maintain bundle integrity. This is a relatively slow process due to the light weight of the polyester yarn.
In general, when a fabric construction is undesirably costly as a function of the fineness of the weave, more yarn ends are added in parallel to the yarn ends extant in the tow bundle. Increasing the number of yarn ends in a tow bundle is also referred to as parallel winding. As a result, there is a higher width to thickness ratio in the tow bundles to be braided. If the higher width to thickness ratio tow bundles are used on the same size machine, the coverage area of the resulting braiding structure increases. Similarly, the higher width to thickness tow bundles can produce a braided structure with the same coverage area on a braiding machine with fewer carriers. Such results are achieved based on a geometric relationship between the aspect ratio of the braiding material, the number of tow bundles or strands used, the braiding angle and the coverage area. This allows for a braided structure of equal coverage to be manufactured on a braiding machine with a reduced number of carriers. The reduction in carrier member is proportional to the increase in the width to thickness ratio achieved by parallel winding. For example, if instead of three yarn ends per stand, seven yarn ends of 500-denier polyester were employed per strand, then a fabric of equal coverage and thickness can be produced on a machine with only 144 carriers.
However, there has been a limit to the number of yarn ends which can be parallel wound in such a way that the yarn ends orient themselves next to one another in order to form a single layer of increased width without additional thickness. Beyond such limit, added yarn ends pile on top of one another or bunch together, increasing both the width and thickness of the resulting tow bundle. Theoretically due to the effects of "textile" catenary (i e., while parallel wound yarn ends are ideally of identical length, practically, variations in length in one or more of such yarn ends result in excess material which can cause bunching; increasing the number of yarn ends increases the likelihood of variations in length among such yarn ends), or other reasons presently unknown in the braiding art, low linear density fibers generally reach such limit at three yarn ends. Additional yarn ends are more difficult to parallel wind, in order to create tow bundles of additional width without additional thickness. Because of this, it is difficult to create a tow bundle using, for example, 500-denier polyester that has an effective width to thickness ratio higher than 3 to 1. For example, winding six yarn ends made of 500-denier polyester in parallel will not consistently yield a tow bundle with an aspect ratio of 6 to 1, it will instead yield a tow bundle with an incidental ratio in the area of about 2.3 to 1.
Hence, the need arises for a tow bundle with a width to thickness ratio that is suitably high to permit the production of a braided structure with a high coverage area while maintaining a small thickness.